gril: Add functions for call-volume refactor

Functions for creating requests and parsing replies needed by
rilmodem call-volume module.
rilmodem: Re-factor call-volume

grilrequest and grilreply functions are used now for creating and
parsing parcels.
unit: call-volume parcels tests
Merge pull request #33 from alfonsosanchezbeato/callvolume-refactor

Callvolume refactor